Clinical microbiology frequently involves the detection of bacteria in bodily fluids. As well as containing bacteria, these bodily fluids may contain cells from the patient themselves. The relative proportions of host and pathogen cells can vary widely e.g. a urine sample may contain many bacteria (>100,000 per ml) but few host cells, whereas a blood sample may contain a large (107 or more) excess of host cells.
Whereas some diagnostic tests can readily distinguish between host cells and bacterial cells (e.g. Gram staining, PCR), others cannot. For instance, some diagnostic tests rely on markers that are also present in host cells, and so the presence of both types of cell can interfere with the test. This sort of interference is found in gingival crevicular fluid (GCF) testing. Proteins such as alkaline phosphatase, acid phosphatase and lactate dehydrogenase have been found to be elevated in GCF samples in diseased states, but all of these enzymes can come from the host or from bacteria [1].
For situations where bacterial cells must be identified within a large background of host cells, but where a non-specific intracellular marker is being used, there is a need for a way of distinguishing the bacterial marker from the host background. More particularly, where a marker of interest within a clinical blood sample could be derived from a blood cell, a bacterial cell, or even from serum, there is a need to distinguish these various sources from each other.